RF Signals are used in a variety of applications, such as medical imaging, broadcast radio, and wireless communications. Many of these signals are modulated, in that the magnitude and phase of a signal changes with time. The specific methods used to modulate signals are varied, resulting in signals that differ in terms of bandwidth, duration, duty cycle, and power levels.
It is often desirable to determine if a modulated signal is present, and if so, the location of the signal emitter. For example, the transmission of an RF signal may be detected as part of a criminal investigation, or to detect and locate unauthorized or unintentional transmissions. The modulation format for some signals may be intentionally chosen to minimize the probability of detection by traditional techniques, such as viewing the spectrum with a spectrum analyzer.
Even when a signal is not designed to avoid detection, a signal may not be detected by traditional techniques if the emitter is located some distance away. Signal power decreases with distance, so detection is dependent on the signal-to-noise (SNR) requirements for the signal detection device and the distance to the signal source. Most detection and geolocation techniques require a positive SNR at the detectors. The techniques may also require a continuous signal or a signal that appears on a regular basis. For example, many direction finding systems can only look in one direction at a time. Consequently, the direction finding system will not locate the transmitter when a signal is present and the directional antennas are pointed in the wrong direction.